The spectrum of frequencies falls off with increasing frequency. To make the runlength encoding efficient, the highest frequencies should be visited last, since there will be a lot of zeros there. The zigzag pattern basically goes through the 2D frequency space from low (starting at 0,0 aka DC) to high.

Answer by Prithvi

Submitted on 3/30/2005

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After we quantize the DCT coefficients,we have lots of zeroes in the matrix.If we use zigzag pattern now,we can have a continous run of zeroes which is capitalized by the Run-Length Encoding.
comments:prithviadhi@yahoo.com

Answer by sam

Submitted on 8/4/2005

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by doing zig-zag scanning all successive zeros (high freq) comes together , which further easy to apply run length coding , where here by doing this , the co-efficients are arranged in low frequency , mid and high -freq

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